Cooling system for an internal combustion engine

ABSTRACT

In a liquid-coolant cooling system for an internal combustion engine, of the kind in which a piping by-passing the radiator and put in parallel with respect thereto, is provided, connected by a three-way valve to the radiator inlet piping, the improvement comprising a special throttling valve for the coolant, installed in the radiator-by-passing piping. The special valve allow the rate of flow of liquid through the radiator-by-passing pipe to be increased as the pressure therein is increased. The advantages of the device are to prevent localized overheatings in the engine block and reducing the emission of unburned hydrocarbons when the engine is running cold.

United States Patent 91 Garcea et a1.

[ COOLING SYSTEM FOR AN INTERNAL COMBUSTION ENGINE [75] Inventors:Giampaolo Garcea, Milan; Dario Radaelli, Legnano, both of Italy [73]Assignee: ALFA Romeo S.p.A., Milan, Italy [22] Filed: Feb. 4, 1972 [21]Appl. No.: 223,440

[30] Foreign Application Priority Data Feb. 5, 1971 Italy 20236/71 [52]US. Cl. 123/41.1, 123/41.13 [51] Int. Cl. F0lp 7/14, FOlp 3/00, F01p7/16 [58] Field of Search 1215/41.], 41.13, 41.08

[56] References Cited UNITED STATES PATENTS 1,253,695 l/19l8 LaPorte123/4113 1,649,248 11/1927 Muir 123/41.13 1,785,207 12/1930 Page123/4108 2,500,472 3/1950 Sohler l23/41.1 2,808,038 10/1957 Scherenberg123/4l.1

[451 Apr. 23, 1974 3,459,161 8/1969 Kolle l23/4l.1

2,622,572 12/1952 Nallinger 123/4l.13 X

FOREIGN PATENTS OR APPLICATIONS 27,127 3/1967 Japan l23/4l.l

Primary Examiner-Al Lawrence Smith Attorney, Agent, or FirmHolman &Stern ABSTRACT In a liquid-coolant cooling system for an internalcombustion engine, of the kind in which a piping bypassing the radiatorand put in parallel with respect thereto, is provided, connected by athree-way valve to the radiator inlet piping, the improvement comprising a special throttling valve for the coolant, installed in theradiator-by-passing piping. The special valve allow the rate of flow ofliquid through the radiatorby-passing pipe to be increased as thepressure therein is increased. The advantages of the device are toprevent localized overheatings in the engine block and reducing theemission of unburned hydrocarbons when the engine is running cold.

8 Claims, 10 Drawing Figures COOLING SYSTEM FOR AN INTERNAL COMBUSTIONENGENE BACKGROUND OF THE INVENTION This invention relates toimprovements in or relating to the cooling system of an internalcombustion engine, of the kind employing a liquid coolant, moreparticularly for motor vehicles.

Systems of this kind usually comprise a radiator, which forms theheat-exchanger for exchanging heat with the surrounding atmosphere, anda non-metering pump, driven by the engine, sends the liquid from: theradiatorto appropriate channels formed in the engine block. The liquidflowing through these channels is brought back to the radiator in such away as to make up a closed loop.

The radiator is proportioned consistently with the maximum amount ofheat which is to be dissipated. In order that the optimum temperature ofthe liquid coolant may be attained within the shortest possible time,and to maintain this temperature in the different conditions of use ofthe engine, a deflecting valve is provided, as controlled by an elementresponsive to the temperature of the liquid emerging from the channelsof the engine block, in the cooling system, the automatic operation ofthis valve thus permits the temperature of the liquid may to bemaintained virtually constant, so that the engine temperature may staywithin an acceptable range, inasmuch as the increase in the resistancein the cooling loop as a consequence of throttling, reduces the rate offlow of the pump. In order to prevent over-pressure build-up and acomplete stagnation of water when the thermostatic valve is closed, ithas been suggested than an auxiliary piping be provided whichpermanently by-passes the throttling valve and the radiator, offeringhydraulic resistance which is greater than that of the radiator, sothat, when the valve is open, the rate of flow in the above mentionedauxiliary piping is not significant as compared with that flowingthrough the radiator.

Such an approach involves serious drawbacks when high powers arerequired ofa cold engine: as a matter of fact, the thermostaticallycontrolled valve cuts off the radiator and the high pressure dropundergone by the coolant as it flows through the auxiliary piping,drastically reduces the rate of flow of the liquid as delivered by thepump: under these conditions, when the engine is running at full load,localized overheating areas may be formed, which originate seriousfailures. On the contrary, if the hydraulic resistance of the auxiliarypiping which bypasses the radiator is decreased, the time which isrequired for the liquid coolant, and thus for the engine, to attain thesteady state temperature, is undesirably extended.

SUMMARY OF THE INVENTION According to the present invention thesedefects have been overcome by causing the rate of flow of the liquidwhich, when the engine is cold, by-passes the radiator, to be increasedas the power delivered by the engine is increased.

It has thus become possible to shorten the time requred by the engine toattain the steady state temperature, local over-heating possibilitiesbeing thus prevented.

A further result as attained by the present invention is to reduce to aconsiderable degree the emission of unburned fractions with the engineexhausts, as compared with the conventional approaches affording similaroverheating prevention features.

The liquid coolant circulation loop according to the present inventioncomprises a first piping which includes a radiator, and a second pipingmounted in parallel with respect to the radiator by the agency of valvemeans controlled by means responsive to the temperature of the liquidcoolant, to deflect the liquid from the first to the second piping when.the temperature is below a preselected value, and is characterized by avalve, placed in the second piping and governed by means responsive tothe power delivered by the engine, the valve being adapted to reduce theflow crosssectional area of said second piping when the engine isrunning at a low power.

According to a preferred embodiment, the cooling loop according to thepresent invention comprises a radiator, a first branch of said firstpiping which connects the liquid delivery side of the radiator to theintake side of a pump whose rate of flow is decreased as the pressuresupplied thereby is increased, said pump sending the liquid to the inputside of the ducts placed in the interior of the engine block, a secondbranch of the first piping connecting the output of the liquid of theengine block to the radiator input, the second piping connecting thefirst branch to the second branch of the first piping, a three-way valvearranged in either of two points of connection of the second piping withthe first piping, the three-way valve being controlled by an elementsensitive to the temperature of the circulating liquid so that theliquid is circulated through the radiator for temperatures above apreselected level, and, for temperatures below another value which issomewhat lower than the first is circulated through the second piping,whereas for intermediate temperatures it is cir' culated partly throughthe radiator and partly through the second piping, there being providedin the second piping a throttling valve for the liquid flowingtherethrough, the valve being controlled so as to vary the flowcross-sectional area of the liquid by the agency of means sensitive tothe power delivered by the engine, so as either to widen or to restrictthe cross-sectional area as the power delivered by the engine is eitherincreased or decreased, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the advantages andfeatures of the cooling loop according to the present invention may bebetter understood, a practical embodiment thereof will be described byway of example only with reference to the accompanying drawings,wherein:

FIG. 1 is a diagrammatical showing of a conventional cooling system;

FIG. 2 is a diagram of the operation of the system of FIG. 1;

FIG. 3 is a diagrammatical showing of an alternative conventionalembodiment of the cooling system.

FIG. 4 is the working diagram of the system of FIG.

FIG. 5 shows the cooling system according to the present invention;

FIG. 6 is the working diagram of the circuit of FIG.

FIG. 7 is a cross-sectional view of a valve of the system of FIG. 5, and

FIG. 8 is a detail of FIG. 7;

FIG. 9 shows the valve of FIG. 7 connected to the throttling valve ofthe engine feed;

FIG. 10 shows a further alternative embodiment for controlling the valveof FIG. 7.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIG. 1,there are diagrammatically shown at l the engine block, at 2 the headand at 3 the oil sump tank of a conventional reciprocating internalcombustion engine, to whose shaft 4 a flywheel 5 is keyed. The engineblock, consisting of the engine crankcase l and the head 2 has a net ofinner channels therethrough (not shown) for circulating the coolingliquid, these channels being connected to a delivery piping 6 and adischarge piping 7. The latter feeds, through a thermostatic valve 8, apiping 9 which opens into a manifold 10 of a bundle of gilled tubes 11which is the member which exchanges the heat with the atmosphere. Thetubes ll, or anyhow the ducts having a high external surface, open intoa manifold 12 from which starts a piping 13 which feeds a pump 14feeding with liquid the piping 6: the whole makes up a closed hydraulicloop.

A piping 15, having a reduced cross-sectional area then connectsdirectly the pipings 6 and 13.

The non-metering pump 14, for example of the centrifugal type, is drivenby the engine, and it is shown in the drawing, for the sake ofsimplicity, as keyed to the crankshaft 4. At any rate, it can beconnected to the crankshaft by any rotational drive, such as by a belt.For simplifying the drawing, further conventional members which completethe cooling system have been omitted, such as the fan which thrusts airthrough the radiator so as to sweep the tubes 11.

The operation of the conventional circuit as shown in FIG. 1 will now bebriefly described, with further reference to the diagram of FIG. 2, inwhich the abscissae indicate the rates of flow and the oridnates thepressures of the circuit of FIG. 1.

As the engine is started at the temperature of the ambient atmosphere,that is when the engine is cold, the valve 8 is closed by the agency ofthermostatic means sensitive to the temperature of the liquid in thepiping 7, that is, the liquid flowing out of the engine.

Under these conditions, the liquid flows through a loop formed by thepump 14, the piping 6, the engine inner channels, the piping 7, l5 and aportion of the piping 13 to the pump again. Due to the presence of thereduced flow cross-sectional area piping 15, the hydraulic resistancesof this path are rather high and shown by the line R, of the plot ofFIG. 2, as a function of the rate of flow Q.

A small permanent rate of flow of liquid through the valve 8, even whenthe latter is closed, permits the thermostatic control means to bemounted within the valve which are thus always swept by the wateremerging from the engine; this rate of flow, however, is whollynegligible.

As the engine has attained an appropriate temperature, the liquidemerging from the engine is also at a high temperature, for example 85C;the thermostatic valve is then opened and the liquid is allowed to becirculated from the piping 7 through the piping 9, the

tubes 11, the piping 13 and the pump 14: the resistence of the loop inhot conditions is shown by the curve R of the plot.

The same plot shows two characteristic curves for the pump, at anaverage rate of rotation, curve s, and at a high rate of rotation, curver.

It is apparent that, in cold conditions, the rates of flow of the liquidpumped through the engine at either an average or a high rate ofrotation, are q and Q8, respectively and, in hot conditions, q and q".respectively. When the valve 8 is only half-way open, the resistance inthe circuit will be intermediate between the curves R; and R, the samebeing true of the rates of flow, for example at an intermediate rate offlow, where they will be intermediate between (1;, and q The valve 8begins to open slightly before the optimum temperature, for example at Cand is fully open at temperatures, for example higher than C.

The principal defect of the cooling loop so embodied has proven to bethe small magnitude of the rate of flow q that is, the rate of flow incold conditions at a high engine rate of rotation. As a matter of fact asmall rate of flow has proven to be insufficient to prevent hazardouslocal overheating whenever a cold engine is required to supply highpower. That is to say, the velocity of the water flowing through theinternal channels of the engine is inadequate to remove the necessaryamount of heat from certain areas. On the other hand, the valve 8 entersaction for increasing the rate of flow, since its thermostatic controlmember is responsive only to the temperature of the water emerging fromthe pipe 7, the water being at an intermediate temperature with respectto that of the different areas of the engine swept thereby. The troubleswhich can be originated by such over-heating, for example incorrespondence of the explosion chambers, are extremely serious and caneven cause jamming or perforation of the pistons.

In order to overcome these drawbacks, a system of the kind shown in FIG.3 has been suggested: parts equal to those of the system depicted inFIG. I have been indicated by equal reference numerals. In this case thepiping 7 is terminated by a three-way valve 16, which isthermostatically controlled and, when hot, establishes a communicationbetween the tubing 7 and the tubing 9 and, in cold conditions,establishes a communication between the piping 7 and a tubing 17 which,in turn, is in communication with 13 and offers to the liquid ahydraulic resistance which is substantially equal to that of the tubes11. In cold conditions, thus, the liquid flows from the piping 7 to thepiping 13 directly via the duct 17 and, in hot conditions, through theducts 9, 10, 11 and 12, though with the same pressure drop in bothcases.

The plot of FIG. 4 shows the operability of the loop describedimmediately above, in the same coordinates as in the loop of FIG. 2. Theresistances in the cold and in the hot are represented by the line R Rand the rates of flow which pass through the engine either at an averageor a high rate of rotation are indicated at q, and q,, respectively. Inthis case, the quick attainment of the steady state temperature by theengine is due only to the fact that, in cold conditions, the liquid doesnot flow through ducts having an intensive heat exchange with theexternal atmosphere. With this second kind of duct it has been madepossible to overcome the defects of the system shown in FIG. 1, but therate of flow has proven to be too high for comparatively reduceddelivered powers, that is, in the great majority of cases, since adriver very seldom uses a newly started engine at full power. It hasbeen suggested that such an excess of rate of flow could give rise to anunnecessary and exceedingly high heat removal at the engine head and theengine barrels and thus remove heat from the gas during the combustionof the mixture. It is known, in fact, that the presence of unburnedhydrocarbons in the engine exhaust gases is essentially due to the layerof mixture which, by sticking to the combustion chamber walls, is keptcold thereby so as to hinder both the propagation of the flame and thecombustion of the mixture layer in question. It is known that thethickness of the cooled layer is greater, the colder the walls, it beingalso known that among the noxious emissions which are set free duringthe operation of a motor vehicle, a not negligible fraction is emittedduring the engine warm-up. Laws now in force set a limit for emissions,ruling that an emission reading be taken during a test with cold enginestart (thus comprising the engine warm up stage). This speculation asbased on theoretical considerations, has been confirmed by the resultsof accurate experimental research which results were that atcomparatively reduced powers (such as prescribed in the various laws)the rates of flow of liquid q of the system of FIG. 3, for beingconsiderably higher than those, q,, of the system of FIG. 1, actuallygave rise to an exceedingly high cooling of the gases during combustion,especially during the engine warm-up stage, so that the total amount ofunburned hydrocarbon emissions from the vehicle (which thus comprisedboth the emissions of a cold and a warm engine) was increased by atleast 10 percent.

As a result of this practical confirmation it has been deemed advisableto study the possibility of providing a cooling system which, with acold engine, might simultaneously prevent both the possible enginefailures at high power delivery, which characterized, as outlined above,the system of FIG. 1, and the high emissions due to an exceedingly highrate of flow of the coolant at low power deliveries by the engine, whichare characteristic of the system of FIG. 3.

According to the present invention, the circulation of water in theengine is increased as a function of the power delivered by the engine.

The cooling system according to the present invention is shown in FIG. 5and equal componentparts are indicated by equal reference numerals withrespect to those already employed for the systems of FIGS. 1 and 3. Inthe present cooling system the piping 17 communicates with the piping 13through a valve, as generally indicated at 18, which brings about avariable restriction of the cross-sectional area, consistently with thevariations of the liquid pressure upstream of the valve.

An example of such a valve 18 is shown in FIG. 7 and consists of a cup19 integral with the piping l7, and in which an obturator 20 is movable,as biassed by a spring 21 which rests against a bottom wall 22. Asclearly shown also by the plan view of the obturator in FIG. 8, when thevlave is closed, as shown in FIG. 7, the liquid flowing through thepiping 17 must also flow through the port 23. When the pressure upstreamof the valve, top portion of FIG. 7, exceeds a certain value, the spring21 is compressed as the obturator 20 drops and the liquid flows alsothrough the open spaces 24 of the obturator.

It is now apparent that the operation of the system according to thepresent invention is the one summarized by the plot of FIG. 6. When theengine is cold and at slow running the hydraulic resistances encounteredby the liquid are those of the first portion of the curve R that is, theport 23 acts like a restrictor with a function similar to that of thepiping 15 of the system of FIG. 1. However, when the engine has reachedsuch a rate of rotation that the pump operates with a characteristiccurve s and thus at a pressure h, the valve 18 begins to open, and iscompletely open as the pump follows the characteristic curve s and has apressure h. The valve resistance drop as the valve is open causes therate of flow to be increased from q to q;,,, the latter beingsubstantially equal to the rate of flow q which obtains in the warmengine, that is, when the valve 16 feeds the piping 9 with liquid.

Thus when the engine is cold the system according to the inventionoperates, at a low rate of rotation of the engine, like the system ofFIG. 1, .and, at a high rate of rotation of the engine, likethe one ofFIG. 3, so that fast warm up of the engine is warranted incorrespondence with the explosion chambers without any risk ofoverheating at high rates of rotation.

It is extremely important to note that the opening of the valve 18, andthus the resistance drop in the piping 17, which by-passes the radiator,is determined by the increase of the rate of rotation of the engine.Actually, it would be advisable that the resistance drop would bestrictly proportional to the power actually delivered by the engine,although it has proven to be sufficient to have the rate of flow of theliquid coolant proportional to other physical units bound to the enginepower, such as, as described above, to the rate of rotation of theengine, or the pressure of the pump when the latter is of thecentrifugal type.

None-the-less other physical units which are a function of the enginepower can be useful for controlling the throttling valve in the piping17: for example the position of the engine feed throttling valve. It isthus possible to arrange in the piping 17 a throttling valve which isdriven open when the throttling valve of the engine feed is open. Thisis obtained by a mere meclianical linkage well-know to persons skilledin the art between the two valves as may be seen in FIG. 9. A rod 25,for example is fastened to the obturator 20 of FIG. 7 and to theaccelerator pedal 28 of the vehicle. The accelerator pedal also controlsthe throttle 32 in the ini take duct of the engine, so that when thepedal 28 is depressed, throttle 32 opens, and element 20 of valve 18 isdisplaced against the action of the spring 21 thus increasing thepassage section for the cooling liquid in theduct 17.

The valve in the piping 17 can still be controlled by the pressureobtaining in the engine intake duct, the pressure being increased, as isknown, when the throttling valve for the aeriform fluid feeding theengine is opened. To this end, it suffices to connect the intake ductwith a chamber having a wall formed by a deformable diaphragm, thelatter being mechanically connected to a throttling valve installed inthe piping 17, in such a direction as to close the latter as the feedingpressure is decreased.

These diaphragm devices which drive a mechanical memberas a function ofthe pressure obtaining in the intake duct are well known and in the artare usually mounted on internal combustion engines in order to actuateeither adjustment mechanisms or servocontrols. Such a device is shownconnected to the valve 18 of FIG. 7, between the valve and theaccelerator pedal of the vehicle. The diaphragm 34 of the devicetogether with the rigid wall 35 define a chamber 36 which is connectedto the intake duct of the engine. When the intake duct pressure is low,the obturator is held against the wall 19 so that the passage section isalmost throttled. However when the pressure in duct 33 is increased theobturator 20 is displaced by spring 21 so that the passage section ofduct 17 is increased.

Other physical units which are a function of the power delivered by theengine can be used for controlling the position of the throttling valvemounted in the cooling system, by the agency of appropriate servoingmembers.

The system according to the present invention affords advantages evenwhen the engine is at its steady state temperature and, however, thevalve 16 is in an intermediate position, that is, the liquid partlyflows through the radiator and partly through the by-pass piping 17. Asa matter of fact, under these conditions, the resistance of thehydraulic circuit has an intermediate value and, likewise, the rate offlow is comprised between the maximum value under hot conditions and theminimum value under cold conditions. At any rate, an abruptincrease ofthe delivered power causes the rate of flow to be immediately increased,preventing overheating which could be experienced if the increase of therate of flow should require of necessity the operation of the valve 16.As a matter of fact, this valve has a certain delay at the beginning ofits operation due both to thermal inertia and the insertion ofthermostatic control means.

Lastly, it is important to note that the three-way valve 16 can merelybe replaced by two distinct valves, one on the piping 9 and the other onthe piping 17, which are driven open and closed, respectively, bythermostatic means acting in opposite directions. In this case thethermostatically controlled valve installed in the piping 17 could beintegral with the valve 18.

In the same way, the position of the pump could 'be varied, by arrangingit in any appropriate point of the hydraulic loop.

Further modifications and changes may be introduced in the systemdescribed above by way of example, without thereby departing from thespirit and scope of the present invention as defined in the appendedclaims.

What is claimed is:

l. A liquid-coolant cooling system for an internal combustion engine,more particularly for motor vehicles, comprising a first piping whichincludes a radiator, and a second piping placed in parallel with theradiator through valve means governed by means sensitive to thetemperature of the liquid coolant for deflecting the liquid from thefirst to the second piping when said temperature is below a preselectedvalue, characterized by a throttling valve placed in said second pipingand governed by means sensitive to the power delivered by th engine,said throttle valve being adapted to reduce the cross-sectional area ofsaid second piping when said power is below preselected value.

2. A cooling system according to claim 1, wherein the hydraulicresistance of said second piping with its throttling valve open, is inthe same order of magnitude as the hydraulic resistance of the firstpiping.

3. A cooling system according to claim 1, wherein a first branch of saidfirst piping connects the output of liquid from the radiator with theinput side of a pump whose rate of flow is decreased as its pressure' isin creased, said pump sending the liquid to the input side of the ductsinternal to the engine block, a second branch of said first piping whichconnects the output of liquid from the engine block with the radiatorinput, said second piping connecting the first branch to the secondbranch of the first piping, a three-way valve arranged in one of thepoints of connection of the second piping with the first piping, saidthree-way valve being driven by an element sensitive to the temperatureof the flowing liquid so that the liquid circulates through the radiatorat temperatures over a predetermined value, and at temperatures belowanother value which is somewhat lower than the first it circulatesthrough the second piping, whereas at intermediate temperatures itpartly circulates through the radiator and partly through the secondpiping, there being provided in said second piping a throttling valvefor the liquid flowing therethrough and driven so as to vary the flowcrosssectional area for the liquid by means sensitive to the powerdelivered by the engine, in the direction as either to widen or torestrict said cross-sectional area as the power delivered by the engineis either increased or decreased, respectively.

4. A cooling system according to claim 3, wherein said pump is of thecentrifugal type and is driven to rotation by the engine at a speedwhich is proportional to the speed of rotation of the engine, saidthrottling valve comprising a movable obturating member, as biassed by aspring, which can be moved from a first position, wherein it leaves saidrestricted cross-sectional area free for the liquid flow, to a secondposition wherein it leaves free for the liquid flow said widercross-sectional area, said obturator being brought from said first tosaid second position by the pressure differential upstream anddownstream of said valve, when said differential exceeds a preselectedvalue.

5. A cooling system according to claim 1, wherein said throttling valveis mechanically connected with the adjusting member for the flow of theaeriform fluid fed to the engine in the sense of reducing thecrosssectional area of said second piping when said member reduces theflow of the aeriform fluid fed to the engine.

6. A cooling system according to claim 1, wherein said engine has atleast one combustion chamber and said throttling valve is driven by adevice responsive to the pressure of the aeriform fluid fed to saidcombustion chambers, in the sense of reducing the crosssectional area ofsaid second piping when said pressure of the aeriform fluid is belowcertain preselected values.

7. A cooling system according to claim 6, wherein said device consistsof a chamber placed in communication with a duct through which theaeriform fluid flows towards said combustion chambers, a wall of saidchamber being displaceable by the agency of the pressure obtaining insaid chamber and being mechanically connected to said throttling valve.

8. A cooling system according to claim 7, characterized in that saidmovable wall is a resiliently yielding diaphragm.

1. A liquid-coolant cooling system for an internal combustion engine,more particularly for motor vehicles, comprising a first piping whichincludes a radiator, and a second piping placed in parallel with theradiator through valve means governed by means sensitive to thetemperature of the liquid coolant for deflecting the liquid from thefirst to the second piping when said temperature is below a preselectedvalue, characterized by a throttling valve placed in said second pipingand governed by means sensitive to the power delivered by th engine,said throttle valve being adapted to reduce the cross-sectional area ofsaid second piping when said power is below preselected value.
 2. Acooling system according to claim 1, wherein the hydraulic resistance ofsaid second piping with its throttling valve open, is in the same orderof magnitude as the hydraulic resistance of the first piping.
 3. Acooling system according to claim 1, wherein a first branch of saidfirst piping connects the output of liquid from the radiator with theinput side of a pump whose rate of flow is decreased as its pressure isincreased, said pump sending the liquid to the input side of the ductsinternal to the engine block, a second branch of said first piping whichconnects the output of liquid from the engine block with the radiatorinput, said second piping connecting the first branch to the secondbranch of the first piping, a three-way valve arranged in one of thepoints of connection of the second piping with the first piping, saidthree-way valve being driven by an element sensitive to the temperatureof the flowing liquid so that the liquid circulates through the radiatorat temperatures over a predetermined value, and at temperatures belowanother value which is somewhat lower than the first it circulatesthrough the second piping, whereas at intermediate temperatures itpartly circulates through the radiator and partly through the secondpiping, there being provided in said second piping a throttling valvefor the liquid flowing therethrough and driven so as to vary the flowcross-sectional area for the liquid by means sensitive to the powerdelivered by the engine, in the direction as either to widen or torestrict said cross-sectional area as the power delivered by the engineis either increased or decreased, respectively.
 4. A cooling systemaccording to claim 3, wherein said pump is of the centrifugal type andis driven to rotation by the engine at a speed which is proportional tothe speed of rotation of the engine, said throttling valve comprising amovable obturating member, as biassed by a spring, which can be movedfrom a first position, wherein it leaves said restricted cross-sectionalarea free for the liquid flow, to a second position wherein it leavesfree for the liquid flow said wider cross-sectional area, said obturatorbeing brought from said first to said second position by the pressuredifferential upstream and downstream of said valve, when saiddifferential exceeds a preselected value.
 5. A cooling system accordingto claim 1, wherein said throttling valve is mechanically connected withthe adjusting member for the flow of the aeriform fluid fed to theengine in the sense of reducing the cross-sectional area of said secondpiping when said member reduces the flow of the aeriform fluid fed tothe engine.
 6. A cooling system according to claim 1, wherein saidengine has at least one combustion chamber and said throttling valve isdriven by a device responsive to the pressure of the aeriform fluid fedto said combustion chambers, in the sense of reducing thecross-sectional area of said second piping when said pressure of theaeriform fluid is below certain preselected values.
 7. A cooling systemaccording to claim 6, wherein said device consists of a chamber placedin communication with a duct through which the aeriform fluid flowstowards said combustion chambers, a wall of said chamber beingdisplaceable by the agency of the pressure obtaining in said chamber andbeing mechanically connected to said throttling valve.
 8. A coolingsystem according to claim 7, characterized in that said movable wall isa resiliently yielding diaphragm.